Modified vinylester resins are becoming more common for many applications. Maleic anhydride and maleic acid modified vinylesters have been reported. For example, U.S. Pat. No. 3,632,861 discloses that improved heat distortion temperatures are obtained from vinylester resins prepared from an unsaturated dicarboxylic acid half ester of a hydroxyalkyl acrylate which is reactive with a polyepoxide or a melamine resin. The improvement results from isomerizing at least about 30 mole percent of the half ester to the trans form before reaction with the polyepoxide.
Also, U.S. Pat. No. 3,773,856 discloses a process for preparing an unsaturated epoxy ester resin composition which can be cured by the addition of an organic peroxide and an accelerator even at room temperature, the process comprising heat reacting an epoxy component selected from the epoxy compounds having at least one glycidyl ether type epoxy group in their molecular structure, with a first acid component from the group consisting of the polymerizable unsaturated monobasic acids of 2-8 carbon atoms and a second acid component selected from the group consisting of the saturated monobasic acids, saturated polybasic acids, saturated polybasic acid anhydrides, polymerizable unsaturated polybasic acids and polymerizable unsaturated polybasic acid anhydrides, in the presence of a polymerization inhibitor and an esterification catalyst in an atmosphere of air or molecular oxygen.
Japanese Patent JP 09077843 82 discloses the manufacture of vinylesters by isomerization of maleic acid to fumaric acid at low temperature. These esters are manufactured by (A) treatment of epoxy compounds with unsaturated monobasic acids, ring-opening addition of maleic anhydride (I) to the OH of the resulting vinylesters, and conversion of the maleic acid (II) residues to fumeric acid (III) residues at a low temperature, or by (B) ring-opening addition of (I) to glycols, conversion of (II) residues in the resulting acid-terminated unsaturated polyesters into (III) residues and a low temperature, and treatment of the terminal acids of the unsaturated polyesters with epoxy compounds and unsaturated monobasic acids.
Great Britain Patent GB 1,139,100 discloses polyhydroxy polyesters, essentially free from unreacted epoxy and CO2 Groups, which were prepared from dicarboxylic acids, ethylenically unsaturated monocarboxylic acids and a diepoxy compound, such as an epoxy resin.
The reaction of maleic anhydride with a secondary hydroxyl group of the vinylester leads to a branched structure, exhibiting high viscosity and the possibility of gelling during production. However, the dependent secondary hydroxyl groups improve the adhesion properties of the resin and increase the number of reactive sites for further modification of the resin.
Maleic acid may lead to a linear resin structure, but it costs more than maleic anhydride and the handling of a powder chemical such as maleic acid is more difficult than melting maleic anhydride. Also, the prior art maleic acid based resins are too high in molecular weight and viscosity to be useful as a low VOC resin.
A hallmark of the current invention is a safer, lower cost process which produces low VOC resins which are predominantly linear in structure.